Variety and environment effects on the dynamics of saponins in lucerne (Medicago sativa L.)

Lucerne saponins are important antinutritional factors, which may hinder the nutritive value of this forage species. Little information exists on possible environmental influences on their accumulation in plants. This study assessed the effect of variety and climatic factors possibly influencing the saponin content. Saponins are glycosides deriving from various sapogenins (aglycone moieties). Being sapogenins a necessary constituent of saponins, saponin evaluation was based on identification and quantification of their aglycones, which are easier to detect than saponins per se. From analytical determinations, medicagenic acid and zanhic acid resulted the most abundant sapogenins, and their variation in concentration was monitored at weekly intervals along the vegetative season of 2 consecutive years. The results suggested the difficulty in outlining a consistent model of saponin accumulation across various harvests within years. Sapogenin concentrations increased from the second to the third year of growth, possibly more as a consequence of the sward age than climatic conditions in the 2 years. The variety Sapko (selected on the basis of a test assessing biologically-active saponins) showed concentrations consistently low across seasons in each year and from year to year, and differed markedly from the two other cultivars. There was a slight, general trend of positive correlation between concentrations and maximum daily temperatures across the year, but the patterns of concentration did not match necessarily those of temperatures, suggesting that these compounds do not have just a cause-effect relationship with summer temperatures as previously reported. While the variety confirmed to be a key factor in the saponin content, the role of other factors, such as temperature, radiation or sward age (inferred from some of the results), is probably dimmed by complex interactions. On the basis of the present findings, some caution could be recommended in feeding animals with lucerne harvested in late summer and, particularly, on aged swards.